1. Field of the Invention
This invention relates to a hydraulic apparatus for an automatic transmission, particularly to a hydraulic apparatus for an automatic transmission that, when a lockup clutch of a torque converter is locked (stuck) in engaged condition, can release the engagement.
2. Description of the Related Art
In an automatic transmission, when a lockup clutch of a torque converter is locked in the engaged condition, it causes the engine stall. Although the engine stall can be avoided if a forward/reverse clutch is disengaged, it makes the engine restart impossible.
The above problem is remarkable when the torque converter is equipped with only one control valve that controls the engagement/disengagement of the lockup clutch and the capacity of engagement (when engaged) by using hydraulic pressure of oil to be supplied/discharged to/from an oil chamber of the lockup clutch and with only one electromagnetic solenoid valve that controls the operation of the control valve.
To cope with it, Japanese Patent Publication No. Hei 2(1990)-5948 proposes a technique to move a manual valve to a predetermined position for forcibly releasing the engagement of a lockup clutch when it is locked in the engaged position.